Having two batteries, one solely for start-up, the other deep cycle -- relay switch?

10-29-2010, 11:16 PM

Hi, I'm new to this and have virtually no experience working with batteries/ignition, but wat I would like to do is have some sort of a relay that in layman's terms tells battery A (my starter battery) to take a breather once the car is started, then have a deep cycle battery B in the trunk give juice to audio/carputer/lights/in-car electronics

I am not quite sure if this is the ideal set-up and whether or not my OEM alternator ('03 mitsubushi lancer oz rally) can handle this extra load, but if so, can anyone here recommend a product i can use to facilitate my request?

No.
All you do is keep the two batteries separated when the vehicle is not charging.

When it is charging, connect the second battery.
I do this with a relay controlled by the alternator's charge light circuit.
People that don't have charge lights, and other suckers that are stupid enough, buy a "smart battery isolator" which is a voltage controlled relay. (IE - voltage is above 12.5V or 12.8V or 13.5V and the batteries are connected; it disconnects below <whatever> voltage (which is lower than the connect voltage); and uses various time delays.)

The practice is to keep your main battery always connected. There is no need to disconnect it. (Only the other battery(s) when not being charged.)

And forget stuff you read about connecting a battery oncethe other is full or nearly charged - that's all colorful bullsh to blow money from suckers.

Search my posts for more details.
Don't forget to have protection (circuit breaker or fuse) and EACH END of the battery interconnect.

PS - you only need an alternator upgrade if it doesn't handle the total load. The extra battery shouldn't change that (each fully charge battery only adds 1-2 Amps - ie, 12-30W).

Comment

why would only stupid suckers use smart isolators? how is running the trigger off the alternator charge light much different then going with +12.8v and above for a trigger? if the alternator dies, there is no way a battery running an engine will stay above 12.8v for even a minute or so... so its not like there is a risk of shortening your fail-safe drive home. am i missing something?

Comment

why would only stupid suckers use smart isolators? how is running the trigger off the alternator charge light much different then going with +12.8v and above for a trigger? if the alternator dies, there is no way a battery running an engine will stay above 12.8v for even a minute or so... so its not like there is a risk of shortening your fail-safe drive home. am i missing something?

i think its the fact that smart isolators are very expensive as opposed to OldSpark's solution which is much cheaper.

im not entirely sure though, waiting on the old guy to give me some info on required materials/pricing.

an additional question (related) - what battery do you recommend for the secondary battery? a deep cycle optima, or a stinger...maybe a kinetik?? is deep cycle the way to go?

Comment

Charge lights are rarely voltage sensitive, they are usually controlled by by the alternator charging whether its output is 1V or 15V.

The problem with voltage sensing isolators is the complexity in avoiding complex situations.

Generally speaking....

They all require minimum on times, minimum off times, and delays before switching on or off (if not included in the on & off time).

Then there is the voltage threshold issue...
Do you want it on at 12.5V or 13.5V?
Should it switch off at 13V or 12.5V?
(Hence why all should have adjustable thresholds - preferably for both else a fixed hysteresis.)

You may not see the need for the above. I do.
But the charge light overcomes all that. It doesn't care about surface charge, extended sags etc - if it is charging, the batteries will be interconnected.

And having analysed problem & failure modes, I too concluded the charge-lamp solution to have far fewer risks & problems.
(EG - one I have at the moment is a charge light that extinguishes as soon as the engine spins, hence joining batteries during cranking. Although that is actually a benefit, it is not the usual practice for smart isolators. But it is easily defeated.)

doncarbone - the charge light circuit is easiest found on the alternator. Unless it is a newer DP type alternator (that interacts with the vehicles EMS), it should have at least one wire - namely the D+ to the charge lamp. (That's in addition to the heavy+12V "B" or "B+" power to the battery and its chassis ground.)
If it is a multi-wire connection, then it's usually labelled "L".
The charge lamp (circuit) often controls electric fuel pumps or fuel cut-off valves in carbied vehicles.
And of course the charge lamp (circuit) goes to a lamp in the dash.

Apart from the batteries and the cable and the 2 breakers/fuses, the cost is that of the relay. IE - $5 for 30A, $15 for 60A-120A; $25 for 200A-400A - noting that a small relay ($5 15A-30A) may be required to drive a BIG relay - the alternator's (regulator's) charge Lamp circuit may not be able to supply enough current for a BIG relay solenoid.

Keep in mind that a "smart isolator" can have the same type of relay, but it adds voltage sensing circuitry (with hysteresis and time delays) instead of using the alternator's signal.
(Isn't it strange - people going to all that trouble to try to determine if the system is charging? It's almost as if they are trying to make money...?)

As to battery types, that depends in part on the system (eg, the main battery is a cranker for cranking; the 2nd battery would be a cranker for audio surges else deep cycle if deep flattening is required); whether external or internal/vented may determine wet/flooded versus sealed AGM/VRLA.
Despite my love for AGMs, I have decided to use two identical deep-cycle wet cells in my 4WD. They are 110AH & ~780CCA, but the 2nd will be mounted under the body (ie, externally). My desire is a solar when long-term stationary; otherwise redundancy, refrigeration, and winching as required. Their $200 was far cheaper than AGM equivalents.

As too my living... LOL - I just applied for the dole (welfare) after several years not working. My last employment was in IT, but I have a long history with vehicles and electrics/electronics. And a longer one for quashing falsities.

Comment

I don't think I've owned a vehicle WITH a charge light in quite some time. They all have 'Not Charging' lights. Aside from that, the cost of the automatic charging relays is not THAT expensive - I think the Blue Sea one runs about $65, is rated for 120Amps, and is very ruggedly built.

Comment

I don't think I've owned a vehicle WITH a charge light in quite some time. They all have ECM generated 'Not Charging' lights, but nothing direct from the alternator. Aside from that, the cost of the automatic charging relays is not THAT expensive - I think the Blue Sea one runs about $65, is rated for 120Amps, and is very ruggedly built.

doncarbone - the charge light circuit is easiest found on the alternator. Unless it is a newer DP type alternator (that interacts with the vehicles EMS), it should have at least one wire - namely the D+ to the charge lamp. (That's in addition to the heavy+12V "B" or "B+" power to the battery and its chassis ground.)
If it is a multi-wire connection, then it's usually labelled "L".
The charge lamp (circuit) often controls electric fuel pumps or fuel cut-off valves in carbied vehicles.
And of course the charge lamp (circuit) goes to a lamp in the dash.

...

As too my living... LOL - I just applied for the dole (welfare) after several years not working. My last employment was in IT, but I have a long history with vehicles and electrics/electronics. And a longer one for quashing falsities.

thanks again for the detailed response.

i'll take a look under the hood before i commit to purchasing any isolators or relays (like the bluesea one Crazi recommended) and see if I can locate the circuit. Is it just a simple soldering required for that? (your basic 60% tin, 40% lead solder with a 45W or so soldering iron?)

Thanks man - sorry to hear you're on welfare that can't be good, or maybe it's awesome...i'm only 20 and still living at home lol which in a way is kind of welfare!

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doncarbone - my alternator is not the 45 year old original (with external regulator) so I just Y-ed into existing interface connections.
Otherwise I normally make intercepts - duplicated male & female bridges with take-offs for what I want. Hence I do not mess the original wiring and I can easily remove or transfer the modification.

Otherwise insulation stripping and solder is the best, though that may be overkill. (Depending on where your tap/junction is etc. A plain wire-wrap may may be good enough - especially if not considered a critical function or if you have some way of determining failure within reasonable time. My rally-car's alligator-clipped jumper leaded washer bottle survived 3 years - until I scrapped the car and saw my temp emergency fix LOL!)

And thanks for the welfare condolences though things aren't that bad - after all, I am NOT living at home. (Geez man, whatcha think I am? I'm way too independent for that. I moved out when I was 28.) [ Home is okay - especially where social services etc are lacking. Or whilst qualifying for the greater income you need to support your family later.... TIP - move out FAST! ] And 20-at-home is IMO ok - especially if studying or training. It's the 27 year old living-at-homes that worry me!
FYI - I chose not to work. I survived on severance pay and then started selling investments before the 2010 crash (which may still be delayed...) and then started borrowing money... and now... Unfortunately I suspect employment is imminent.

I don't think I've owned a vehicle WITH a charge light in quite some time.

Obviously my solution is only for those with charge lights. (I usually include "not DP" and "not stator" types etc when being verbose - to help make it obvious.)

In other cases, voltage sensing is the only solution even if (generally) not as elegant performance wise - though the "not charging" can also be like the charging light. (Probably with a cranking cut-out or tacho signal. That is still probably better than voltage sensing.)

The current rating of whatever system is used doesn't matter - just drive a relay/contactor of the required size. Many prefer to keep the sensor separate from the relay anyhow.

Addendum 27Aug11:
In case not clarified in this thread, re CraziFuzzy's "I don't think I've owned a vehicle WITH a charge light in quite some time."...
A charge-light is really a "not charging" light - it goes off when the alternator is charging (and hence compatible with the system I describe - now aka the UIBI.)
Nevertheless, any indication of charging status can be used. It's usually just a matter of swapping the UIBI (relay's) coil's other end from gnd to +12V.

Comment

I run two batteries in my 2001 Camaro SS, there is a red top under the hood for the car and a red top in the trunk for the stereo. I wanted to run the stereo while the car was off without draining the battery under the hood so I did some thinking and this is the system that I came up with:

One under the hood, connected to the one in the trunk with a 4guage Power AND ground, the power wire was fused at both ends within 12" of the battery. In the trunk I used a Stinger battery isolator controlled by a DEI delay relay setup. The isolator disconnects the batteries while the car is off then 30 seconds after the ignition is turned on (time to start the car) the two batteries are connected. Works like a charm for me...